1. Field of the Art
The present embodiments relate to paper and paper making.
2. Background
Paper sheets are made by dewatering a pulp suspension, forming a uniform web, and drying the web. Pulp suspensions often contain large amounts of anionic substances including small fiber fines, inorganic fillers, hydrophobic pitch particles, and contaminants from waste paper recycling. Therefore, retention chemicals are commonly added to the pulp suspension to fix the anionic substances to the final paper sheet. In addition, retention chemicals accelerate the pulp dewatering process, resulting in a higher paper production rate.
One of the widely applied retention programs employs a combination of a high molecular weight anionic flocculant and a low molecular weight cationic coagulant. Typical commercial anionic flocculants are copolymers of acrylic acid and acrylamide prepared either by inverse emulsion polymerization or by solution polymerization. Common commercial coagulants are poly(diallyldimethylammonium chloride), polyamines prepared from dimethylamine, ethylene diamine, and epichlorohydrin, alum, polyalluminum chloride (PAC), cationic starch, vinylamine-containing copolymers, and polyethylenimine (PEI). It is generally accepted that coagulants can deposit on the anionic surfaces of various substances and generate cationic patches. Afterwards, the high molecular weight anionic flocculants can bridge cationic patches, increasing the fixation of fines and fillers.
Recently, the water systems in papermaking mills have become ever more closed. This trend leads to an increase of dissolved and suspended solids, such as salt and anionic substances. Water chemistry plays a major role in the effectiveness of a retention program. Salt and anionic substances often interfere with the interactions between retention chemicals and various substances in the pulp suspension, reducing the effectiveness of the retention program. In addition, a reduction in retention efficiency leads to a further increase of dissolved and suspended solids. Consequently, there is an increasing demand for a more effective retention program.
Glyoxalated polyacrylamide (GPAM) is a common temporary wet strength resin. GPAM is typically prepared by reacting glyoxal and a cationic polyacrylamide base polymer (for example, as discussed in U.S. Pat. Nos. 3,556,932, 4,605,702, and 7,828,934, each of which is incorporated herein by reference). GPAM is typically added in the pulp suspension before paper sheet formation. Upon drying of the treated paper sheet, GPAM is believed to form covalent bonds with paper cellulose to increase paper dry strength. Since the covalent bond between GPAM and cellulose is reversible in water, this wet strength may decrease over time. GPAM strength performance also can be adversely affected by relatively high pH and high levels of alkalinity when present as bicarbonate ions.
The description herein of certain advantages and disadvantages of known methods and compositions is not intended to limit the scope of the present disclosure. Indeed the present embodiments may include some or all of the features described above without suffering from the same disadvantages.